The present invention relates generally to the use of coring systems utilized in the oil and gas industries for the determination of material properties in subterranean rock formations; and more particularly relates to the use of cases obtained from predetermined directions proximate a wellbore to determine non-isotropic formation properties.
Anisotropy in rock properties results from inherent characteristics such as crystal orientation, microcrack distribution, and other microgeometric properties. It is well established that rock properties show a significant optional variation within the bedding plane as well as in the vertical direction perpendicular to the bedding plane. The evaluation of anisotropic properties within a formation has been recognized as a valuable aid in designing fracturing programs. Such evaluation is disclosed in U.S. Pat. No. 5,272,916, issued Dec. 28, 1993 to Blauch et al., the specification of which is hereby incorporated herein by reference.
For many applications a transversely isotropic model has been applied which assumes that rock properties within the bedding plane are isotropic. This model is unrealistic for many materials, particularly in the case of fine grained formations such as shales in which microscopic anisotropy within the bedding plane can be quite significant.
Prior art solutions to the problem of the determination of material properties in the bedding planes of subterranean rock formations have included the use of sidewall coring tools to obtain a single sample for a given bedding plane. Examples of such prior art sidewall coring tools are known to those skilled-in-the-art. The use of an exemplary tool is described in U.S. Pat. Nos. 5,105,894, issued Apr. 21, 1992 to Enderlin, and 5,253,719, issued Oct. 19, 1993 to Blauch et al; both assigned to the assignee of the present invention. The disclosures of each of those patents is hereby incorporated herein by reference for all purposes. Such tools have typically been utilized, however, to obtain only a single core sample at a given depth in the formation. It is not apparent that any effort has previously been made to obtain multiple core samples of essentially the same depth but at known azimuthal orientations, or to further use such core sample data to determine non-isotropic formation properties. Thus, the prior art solutions have been confined to a directionally isotropic model of material properties in such bedding planes.
The prior art solutions to the problem of the determination of material properties in subterranean rock formations have not included the use of a plurality of laterally spaced, directionally oriented core samples to thereby provide an accurate prediction of the material properties in a bedding plane as a function of the directional orientation parallel to the bedding plane.
The present invention overcomes the problem of the determination of material properties in subterranean rock formations by permitting the determination of anisotropic material properties in bedding planes of such subterranean rock formations by use of a sidewall coring tool to obtain radially oriented, and preferably generally equally angularly spaced core samples to thereby provide an accurate prediction of the material properties as a function of directional orientation within a bedding plane.